The purpose of this project is to investigate the genetic regulation of the galactose catabolic enzymes, galactokinase (GALK), galactose-1-phosphate uridyl transferase (GALT) and UDP galactose-4-epimerase (GALE) in a mammalian cell culture system. GALK and GALT activity has been determined in both normal and SV-40 transformed human fibroblasts grown in hexose-free medium supplemented with glucose or galactose. It was found that growth in galactose causes a 1.5 to two-fold increase in both of these enzymes. This increased enzyme activity is reflected in an increase in the amount of 14C-galactose which is taken up by the cells and in differences in 14 C-labelled metabolites which accumulate. These results will be confirmed and extended. The galactose biochemical pathway is of special clinical importance because of the genetic diseases associated with deficiencies in either the kinase or transferase enzyme. The effects of these enzyme deficiencies on the other enzymes in the pathway will also be investigated in cells deficient in only one enzyme and in hybrids formed between kinase and transferase deficient cells. Elucidation of the mechanisms responsible for the genetic regulation of the galactose catabolic enzymes will be important in increasing our general understanding of how enzyme amounts are controlled and will provide basic information important to the study of galactosemia.